Friction materials utilized as the lining in drum brake shoes of drum brakes and in the brake pads in automobile disk brakes and other devices (for example in clutch plates) are manufactured with a compound (mixture) comprising a fibrous or fiber material, an organic binder (usually a phenolic synthetic resin) and a bulk or “filler”. Instead of asbestos as the fiber material, which has been legally banned as a substance dangerous to the environment, mixtures of other organic and inorganic materials are utilized, such as rock wool, aramid and carbon fibers, metal fibers or powder such as copper, tin, iron, aluminum, and other metals or metal alloys such as bronze or brass. EP1227262 for example, indicates the use of a friction material of the above described type containing approximately 10% copper fibers by volume, between 0.1 and 15% tin and/or tin sulfide by volume, and between 4 and 9% bronze fibers by volume.
NAO compounds, due to their composition, have an electric conductivity not sufficient to ensure a good painting thereof; things are even worse if the copper is eliminated (due to environment care compounds without copper are more and more requested).
Even though in brake pads the compound is manufactured as a plaque or pad assembled on a metal plate in order to form the actual brake pad, painting brake pads made with non electrically conductive materials presents numerous problems. In particular, it is presently impossible to utilize currently used powder coating plants built to coat/paint brake pads that are electrically conductive.
More generally it is also known that in order to powder coating (a painting technology requiring use of electrostatic charges) non electrically conductive elements such as for example mechanical components manufactured with plastic polymers, a conductive primer is applied to the surface of the non conductive element to be painted. However, conductive primers are based on organic solvents which are noxious and dangerous to the environment.
Attempts to powder coat brake pads made with non conductive compounds have not been successful up to now; this is because, when at all possible, (thanks to the presence of the metal base support for the plaque in a non conductive compound), the preparation, finish, and thickness are not satisfactory: brake pads manufactured using this method are therefore unable to pass standard corrosion tests.
The only alternative available therefore is the use of other coating technologies, which are however more expensive and require large specially devised plants that call for an unacceptable investment considering present manufacturing volumes.